This invention relates to retrofocus type photographic objectives, and more particularly to a compact retrofocus objective having a wide angular field coverage and a longer back focal distance than 1.4 times the focal length thereof.
Recent developments in interchangeable objectives for use in 35 m/m single lens reflex cameras are directed to compact designs, and this tendency is especially pronounced in the wide angle lens art. Because it allows a sufficiently long back focal distance, the retrofocus type has been widely accepted in designing such wide angle objectives. The employment of the retrofocus type, however, gives rise to many difficult problems of aberrational correction because of the asymmetrical lens arrangement.
Of the various aberrations, the distortion which is introduced by the divergent front lens group constitutes a most important subject for correction. According to the generally accepted proposal, a positive component is arranged in the front group of the lens system. With such construction and arrangement of the front lens group, however, the off-axial rays of light are strongly refracted by that positive component. Hence the other aberrations tend to be increased and the diameter of the front member must be increased to secure the equivalent angular field coverage.
As the dimensions of this type wide angle objective are reduced, therefore, these problems are made more difficult to solve, resulting in a less standard of correction of the aberrations. A high standard of aberrational correction has so far been realized by employing especial optical glasses in making the various lens elements. This leads to an increase in the production cost of objectives.
Attempts have been made to provide a compact retrofocus wide angle objective which has overcome the above mentioned drawbacks and which is well corrected for the various aberrations as in U.S. patent application Ser. No. 892,422 (Filed Mar. 31, 1978) assigned to the assignee of the present invention. Of the examples of the lens systems described therein, the fourth example nears the construction of the lens of the present invention. The lens in this example has an angle of view of 75.degree., and f-number of 1:2.0. The present invention provides a retrofocus type lens having a larger angle of view than that of the above mentioned cited example, while nevertheless maintaining the length of the entire lens system at minimum, and achieving good correction of the aberrations.